Freshwater and terrestrial ecosystems are tightly linked by food web
interactions. Naturally occurring carbon and nitrogen stable isotopes provide a tool
to quantify nutrient flows across ecological boundaries, however their application to
freshwater-terrestrial systems has been limited. This thesis evaluated whether stable
isotope analysis can be effective in differentiating freshwater and terrestrial
vascular plants at the base of subarctic food webs and found freshwater plants to be
consistently enriched in their isotopic signatures relative to terrestrial plants.
Stable isotope approaches were then used to investigate spatial and temporal
variability in the diet of a population of subarctic beavers (Castor canadensis).
Freshwater macrophytes contributed more to the diets of beavers than previously
reported. During the winter, beavers from pond habitats consumed more aquatic
vegetation than beavers from stream habitats which relied more heavily on food hoards
of terrestrial vegetation. Aquatic foraging may enable beavers to persist at the
northern periphery of their range by reducing foraging pressure on the subarctic
terrestrial ecosystem where their preferred tree species are scarce.